The use of external selective feedback for spectral narrowing and tuning of single semiconductor lasers started in the early 1970s and was described in numerous original publications and in several classical monographs such as K. Petermann, “Laser Diode Modulation and Noise”, 1988 and L. Coldren, S. W. Corzine “Diode laser and Photonic Intergrated Circuits”, 1995. After development of laser diode stacks, the same methods have been adapted to such multichannel devices and a number of recent experiments have demonstrated improvements of spectral width of broad area laser diode bars.
Two technical approaches were used, an optical injection method and an external cavity technique with selective spectral elements in a feedback loop. Most interesting and promising results for the use of injection wavelength locking and beam quality improvements were recently observed in Oak Ridge National Laboratory (see Y. Liu, H. K. Liu, Y. Braiman, “Injection locking of individual lasers in an integrated high-power diode array”, Appl. Phys. Lett. 81 (2002), pp. 978-980. Using a 0.5 mW injection beam from a single-mode and single-frequency LD to each 125 μm-wide emitter in a bar consisting of 19 diodes, up to 1 W power with a narrow spectrum from each emitter was achieved. At this power, the line width of 8 MHz was concentrated near 60% of radiation. The remaining energy was distributed continuously in a wide spectrum similar to that of a free running laser. For a pumping stack, such precise spectral narrowing and stabilization are even better than necessary. However, significant a part of the radiation outside of the narrow spectral region is not acceptable for highly efficient pumping systems. Additionally, injection wavelength locking meets some serious problems from the point of view of compact, cheap and reliable design, which is necessary for pumping stack.
In a classical optical design, a spectrally selective feedback with a surface diffractive grating was used for narrowing and stabilization the spectral width of laser bar. Theoretically, it was demonstrated that for a stabilized LD source with an external cavity, the requirement of a narrow line width is not in a contradiction with the requirement of maximum efficiency. (See V. Annovazzi-Lodi, S. Merlo, N Moron in “Power efficiency of a semiconductor laser with an external cavity”. Opt. and Quant. Electronics 32 (2000), pp. 1345-1350). After 6 years of experiments with spectral stabilized laser diode stacks, the state-of-the-art efficiency did not exceed 60-65% compared to the efficiency of a free running laser under high pumping conditions. This result was demonstrated by B. Chann, I. Nelsonand T. G. Walker in “Frequency-narrowed external-cavity diode-laser-array bar”. Opt. Lett. 25 (2000) 1352-1355. In their experiments a fraction of the output power in the narrowed peak decreased with increasing current from 85% near threshold pumping current to 68% under high pumping conditions. Maximum output power in a narrow spectral line (60 GHz) run up to 12.2 W for 20 W laser diode bar in an external cavity configuration. For design of the external cavity, the authors used Littrow or Littman-Metcalf optical schemes with blazed surface grating and in both case they achieved similar experimental results. Decreasing of optical power in narrow line under high pumping condition is explained by the thermal distortions of the surface grating which resulted in additional optical losses. Similar grating behavior was observed in spectral combining experiments and plays a significant role for illumination of metallized surface grating by power beam with energy near 10 W or higher. (See S. J. Augst, A. K. Goyal, R. L. Aggarwal, T. Y. Fan, and A. Sanchez. Wavelength beam combining of ytterbium fiber lasers. Optics Letters 28 (2003) 331-333). However, it is shown in the spectral combining experiments with Yb-doped fiber lasers that the volume PRT grating is absolutely stable under illumination at least one order of value higher and has very low absorption losses. (See (L. B. Glebov, V. I. Smirnov, C. M. Stickley, I. V. Ciapurin. New approach to robust optics for HEL systems. In Laser Weapons Technology III. W. E. Tompson and P. H. Merritt, Editors. Proceedings of SPIE, 4724 (2002) 101-109). This data provides work in a wide range of laser diode-stack output powers with high initial efficiency in narrow spectral lines and allows for designing simple and reliable external cavity with transparent volume diffractive gratings in PTRG.
Recently, the use of photo-thermo-refractive volume diffractive gratings for spectral and angular selection in semiconductor lasers were described in (B. L. Volodin, S. V. Dolgy, E. D. Melnik, E. Downs, J. Shaw, V. S. Ban. Efficient technology for wavelength stabilization and spectrum narrowing of high-power laser diodes and arrays. Advanced Solid State Photonics, 2004).
However, prior art publications fail to teach an adequate level of efficiency of laser devices or a useful narrowing of output frequencies at requisite high levels of pumping.